The invention relates to a fully hydraulic brake power generator/master cylinder unit according to the precharacterizing clause of claim 1.
In contrast to the brake booster/master cylinder units still predominantly used today, in fully hydraulic brake power generator/master cylinder units the brake power is not boosted or generated with the aid of a vacuum, but purely hydraulically. In a similar way to conventional vacuum brake boosters, the actuating force normally introduced via a brake pedal by the vehicle driver is boosted proportionally, in that hydraulic fluid is supplied under pressure in a controlled manner by a valve to a booster stage which acts on the pressure chamber or pressure chambers of the master cylinder and thus builds up in the latter the brake pressure which is then transferred to the associated wheel brakes via the individual brake circuits of the vehicle brake system. When the brake pedal is released, this booster stage is then relieved of pressure again under the control of said valve.
In particular embodiments of fully hydraulic brake power generator/master cylinder units, normally, that is to say when the booster stage is functioning properly, their input member is no longer used for introducing power into the unit, but, instead, is employed only for actuating the unit, in that the input member is merely displaced, for example by the depression of a brake pedal connected to the input member. The displacement of the input member is sensed, and a corresponding hydraulic pressure is built up in the booster stage as a function of this displacement, in order to generate the brake power desired by the driver. In this case, the hydraulic pressure emanates from an external source, for example from a pressure accumulator or from a rapid-response hydraulic pump. Only when the hydraulic booster stage fails can the vehicle driver couple the input member mechanically, normally to the primary piston of the master cylinder, by vigorously depressing the brake pedal, so as then to build up at least some brake pressure by depressing the brake pedal further and by means of the resulting displacement of the primary piston.
Since the vehicle driver therefore normally actuates the brake power generator/master cylinder unit only indirectly, the unit itself, in contrast to brake power generator/master cylinder units conventional hitherto, does not give him any feedback on the brake pedal with regard to the brake pressure which has been built up in the unit, thus making it difficult to actuate the brake pedal in a metered manner.
In order to improve the meterability of the brake system in brake power generator/master cylinder units without direct brake pressure feedback to the brake pedal, it is known to simulate brake pressure feedback, for example by means of a spring arrangement, so that the customary behavior of a conventional brake booster/master cylinder unit, in which higher brake pressures require a higher pedal pressure, is imparted to the driver artificially. However, the problem of the known arrangements for artificially generated brake pressure feedback is that, when the hydraulic booster stage is not functioning, for braking purposes the spring force serving for artificial brake pressure feedback to the brake pedal first has to be overcome by the driver, before it is possible to build up brake pressure by a correspondingly more pronounced actuation of the brake pedal. On the basis of this behavior of the brake system, the driver may be given the feeling that his efforts are in vain and may therefore think, incorrectly, that the brake system is entirely inoperable.